Permanent magnets and hard magnetic materials

This review deals with the definition and development of hard magnetic materials with special emphasis on more recent developments in this field. After discussing the fundamental properties of all hard magnetic materials, based on the intrinsic and extrinsic properties, some remarks on the history of hard magnetic materials and permanent magnets are given. The important hard ferrites are only briefly treated. Emphasis, however, is laid on the rare earth (RE) intermetallics and the hard magnetic materials based on them. Experimental techniques for the measurement of the basic properties of hard magnetic materials, e.g. the ordering temperature, the magnetization, the anisotropy field, the physical and technical hysteresis loop and the properties, derived from this loop are discussed. Examples of modern permanent magnets, based on RE–cobalt alloys, on RE–Fe–B alloys and RE–Fe–nitrides, etc are explained. The principles of the production of sintered and bonded magnets are given. Finally some typical applications of these modern permanent magnets are discussed with respect to the different properties of the materials. Future developments and the possibility of superconducting permanent magnets are briefly discussed. Concluding remarks are concerned with the possibilty and necessity of still larger energy products at elevated temperatures and the benefits of high-tech permanent magnets for modern and energy saving devices.